Residuary comes from Latin residuum ('a remainder') + -ary, from residuus ('remaining'), from residere ('to remain, settle behind'), from re- + sedere ('to sit'). The historical chain includes Latin residuus, Latin sedere, and Proto-Indo-European *sed-. The quoted older senses, "a remainder", "remaining", "to remain, settle behind", and "to sit", supply the semantic anchors for residuary. The formation is best read as suffixal, compound, and borrowed: the word residuary carries its older form-history into a compact English lexical shape. In present English it is used as a adj, "relating to the part of an estate remaining after debts". Related English forms include residue, residual, residuum, and reside. The result is a adj whose etymology preserves both the older form and the later English specialization.