The nanosyntax research programme allows spelling out of a larger chunk of syntactic structure by a single morpheme. You can think of it as: "in nanosyntax, each morpheme is a syntactic idiom". Talking about this requires a new view of the spellout mechanism, and some terminology which you might not be familiar with. Here are some key terms, starting from the core terms and adding some more specialised hypotheses below:
Phrasal spellout.
The idea that any node in the tree can correspond to
a lexical item. Put differently, spellout applies to syntactic phrases,
and syntactic phrases are the things that are stored in lexical entries.
(This is a restrictive vew of spanning - spanning only targets
constituents - and is opposed to the more liberal 'sequential spanning',
see below). Phrasal spellout has its origin in Starke's work on -ABLE
and -ED.
The Superset Principle.
The idea that a lexical item can spellout
syntactic structures which are smaller than that lexical item. More
precisely, the structure contained in the lexical item can be bigger
than the structure that it lexicalises. In such cases, the lexical item
is a superset of the syntactic structures being spelled out. This means
that one given lexical item can in principle spell out a range of
different syntactic structures (as long as it is bigger or equal to
those syntactic structures)
Span / spanning.
A lexical item is said to span a syntactic structure if
it corresponds to a continuous sequence in that structure. 'Spanning' is
a cover term for two more specialised hypotheses competing with each
other: sequential spanning and phrasal spellout (or 'phrasal
spanning')
Sequential spanning.
The hypothesis that you can spellout an arbitrary
stretch of the syntactic structure, as long as it forms a continuous
stretch. This means that you can spellout non-constituents. This is in
opposition to "phrasal spellout", see above. This hypothesis is found in
the work by Abels/Muriungi on the Kitharaka focus marker, and has its
origin outside nanosyntax, in Williams' 2003 work. (An even earlier
remote cousin is Brody's mirror principle work, though Brody's notion is
more restrictive than sequential spanning in that it is structural. It
is in fact intermediate between phrasal spellout and sequential
spanning)
Underassociation.
Underassociation is mostly another term for the
Superset Principle. More precisely, you can think of it as the result of
applying the superset principle non-vacuously: when part of the
structure in a lexical item is unused for the spellout of a given
syntactic structure, that unused piece is "underassociated".
Here are some more specialised terms floating around in the nanosyntax
world:
The *ABA.
Informally: if a given span is lexicalised by A, and a
slightly bigger version of this span is lexicalised by B, then it is
impossible for A to lexicalise a span even bigger than B. This
generalisation is formulated by Bobaljik, and it follows from the
superset principle operating on an ordered fseq (functional sequence of
features). Caha's work on the nanosyntax of case makes extensive use of
the *ABA theorem to deduce the structure of case from the syncretisms
found in case systems.
The Identification principle.
This is a hypothesised restriction
constraining the superset principle. The idea is that no piece of the
lexical item can be truly missing. If a piece appears to be missing, it
is because it is lexicalised by some other lexical item. The metaphor is
that the missing pieces have to be "identified" by some other lexical
item in order to be legitimately missing. This hypothesis is found in
Ramchand's book "First phase syntax"
Maximise span.
When a given syntactic node could be spelled out either
by two smaller lexical items or one bigger lexical item, the bigger one
wins. The intuition behind the name is that the bigger one has a bigger
span, so the one with the maximal span ends up winning. (This is also
called 'overriding', see below). This competition principle derives
Poser's generalisation.
Minimise Junk.
When two lexical items are in competition to spellout a
given span/subtree (because they both contain that span/subtree), the
one which wins is the one which contains the least unused material, ie
the least junk. This principle is sometimes also called "best fit": the
lexical entry which contains the span/tree fitting most tightly the
target syntactic structure is the one which wins
Overriding.
In a cyclic bottom-up spellout, spelling out a higher
constituent 'overrides' any earlier spellout of a smaller subpart of
that constituent. This is a derivational way of deriving the 'maximise
span' principle without postulating any principle other than the
derivation itself
Pointers.
In some cases, a lexical item contains another lexical items
(or 'refers to' another lexical item). Idioms are clear cases of this:
the whole idiom is a lexical item (since it is associated to an
arbitrary meaning), but its constituent words are independently existing
lexical items. The lexical entry for the idiom is said to contain
'pointers' in the nodes corresponding to the independently existing
words in the idiom. Thus the lexical entry for 'kick a bucket' will have
a pointer in the N/NP projection , which points to the independently
existing lexical entry for 'bucket'