support vs underpin

support is a noun but underpin is not a noun.

Word	Noun	Adjective	Verb	Adverb
support	Yes	No	Yes	No
underpin	No	No	Yes	No

As verbs, underpin is a hyponym of support; that is, underpin is a word with a more specific, narrower meaning than support:

Other hyponyms of support include scaffold, block, carry, chock, buoy, buoy up, pole, bracket, prop, prop up, shore, shore up, truss, brace.

As verbs, support and underpin are synonyms defined as:

support and underpin: support with evidence or authority or make more certain or confirm

Other synonyms of support include bear out, corroborate.

support (noun)	underpin (noun)
aiding the cause or policy or interests of
the act of bearing the weight of or strengthening
documentary validation
a military operation (often involving new supplies of men and materiel) to strengthen a military force or aid in the performance of its mission
the activity of providing for or maintaining by supplying with money or necessities
any device that bears the weight of another thing
supporting structure that holds up or provides a foundation
something providing immaterial assistance to a person or cause or interest
a musical part (vocal or instrumental) that supports or provides background for other musical parts
the financial means whereby one lives
financial resources provided to make some project possible

support (verb)	underpin (verb)
establish or strengthen as with new evidence or facts	support from beneath
put up with something or somebody unpleasant	support with evidence or authority or make more certain or confirm
adopt as a belief
argue or speak in defense of
be a regular customer or client of
be the physical support of; carry the weight of
play a subordinate role to (another performer)
support materially or financially
give moral or psychological support, aid, or courage to
be behind; approve of
support with evidence or authority or make more certain or confirm