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From The Collaborative International Dictionary of English v.0.48:
Underpin \Un`der*pin"\, v. t. [imp. & p. p. Underpinned; p. pr. & vb. n. Underpinning.] 1. To lay stones, masonry, etc., under, as the sills of a building, on which it is to rest. [1913 Webster] 2. To support by some solid foundation; to place something underneath for support. [1913 Webster]