Green’s Dictionary of Slang

1. (also bosom chum) a louse.

c.1717		‘The Cavalcade’ in A. Carpenter Verse in Eng. in 18C Ireland (1998) 105: Taylors to creeping Louse Eternal Foe: / Nor Bosome-Friends, nor Backbiters they spare.
1738		Swift Polite Conversation 29: (Neverout scratches his Neck) miss.: I beg Pardon for the Expression; but I’m afraid your Bosom Friends are become your Backbiters.
1849		Sam Sly 28 Apr. 2/1: Mr. W—s, pork butcher[...] not allow them [i.e. his ‘shopmen’] to sleep on beds that are full of vermin; you cannot expect your shopmen can rise at five in the morning if they are kept awake by bosom friends.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 33: Bosom Chums: Vermin.
1949		Monteleone Criminal Sl. (rev. edn) 31: bosom chums Fleas; mites; lice.
1960		(con. 1950-1960) R.A. Freeman Dict. Inmate Sl. (Walla Walla, WA) 16: Bosom friends – body lice.

2. (US) a pack of money kept for security inside her brassiere by a woman while she is travelling.

1973

in DARE.