"tenfold, containing ten times as many," 1610s, from French décuple (late 15c.), from Latin decuplus "tenfold," from decem "ten" (from PIE root *dekm- "ten") + -plus (see -plus).
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dasa, Avestan dasa, Armenian tasn, Greek deka, Latin decem (source of Spanish diez, French dix), Old Church Slavonic deseti, Lithuanian dešimt, Old Irish deich, Breton dek, Welsh deg, Albanian djetu, Old English ten, Old High German zehan, Gothic taihun "ten."
-plus
word-forming element, Latin -plus "-fold." Watkins derives it from *-plo-, combining form of PIE root *pel- (2) "to fold" and makes it cognate with Old English -feald, Greek -paltos, -plos. But de Vaan connects it to PIE root *pele- (1) that yielded words meaning "much, many, more" and is the source of poly-.
*pel- 2
Proto-Indo-European root meaning "to fold."
It forms all or part of: aneuploidy; decuple; fold (v.); -fold; furbelow; haplo-; hundredfold; manifold; multiple; octuple; polyploidy; -plus; quadruple; quintuple; sextuple; triple.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit putah "fold, pocket;" Albanian pale "fold;" Middle Irish alt "a joint;" Lithuanian pelti "to plait;" Old English faldan "to fold, wrap up, furl."