Green’s Dictionary of Slang

1. fat, fatness.

[1846		Swell’s Night Guide 58: I takes my pitch last night on Fleet pave, then meets with Bet Flab, the Yarmouth bloater].
1923		Glasgow Herald 15 Nov. 8: Other terms in every day use [at Christ’s Hospital] are ‘flab’, butter [etc.] .
1959		A. Sinclair My Friend Judas (1963) 107: His flab sagged down, his belly sat in a mooning bulge pathetically upon his thighs.
1969		N. Cohn Awopbop. (1970) 102: Maybe he did have flab problems.
1978		A. Maupin Tales of the City (1984) 139: She grabbed a handful of fanny flab.
1999		N. Cohn Yes We Have No 317: When the last flab has been burned off.
2001		Guardian G2 20 Mar. 18: Muscle, rather than flab.

2. (UK juv.) a fat person.

1959

I. & P. Opie Lore and Lang. of Schoolchildren (1977) 188: Flab, football, glutton, grub-tub.