What is the X in X-mas?
Old English fif, from Proto-Germanic *fimfe (cf. Old Frisian and Old Saxon fif, Dutch vijf, Old Norse fimm, Old High German funf, Gothic fimf), from PIE *penkwe- (cf. Sanskrit panca, Greek pente, Latin quinque, Old Church Slavonic peti, Lithuanian penke, Old Welsh pimp). The sound shift that removed the *-m- is a regular development involving Old English, Old Frisian, and Old Saxon (cf. thought, from stem of think; couth from *kunthaz; us from *uns.
Slang five-finger discount "theft" is from 1966. Five o'clock shadow attested by 1937. The original five-year plan was 1928 in the U.S.S.R.