An example of underpin is for beams to hold up a tree house.
transitive verb-·pinned′, -·pin′ning
- to support or strengthen from beneath, as with props
- to support or strengthen in any way; corroborate, substantiate, etc.
transitive verbun·der·pinned, un·der·pin·ning, un·der·pins
- To support from below, as with props, girders, or masonry.
- To give support or substance to: assumptions that underpinned their claims.
(third-person singular simple present underpins, present participle underpinning, simple past and past participle underpinned)
under- +"Ž pin.