Green’s Dictionary of Slang

bender! excl.

[slightly predates over the bender under bender n.1 but prob. same root; note WWI RN bender, a yarn, a tale]

nonsense! humbug! rubbish!

[UK]Vaux Vocab. of the Flash Lang. in McLachlan (1964) 226: bender: an ironical word used in conversation by flash people; as where one party affirms or professes any thing which the other believes to be false or insincere, the latter expresses his incredulity by exclaiming bender! or, if one asks another to do any act which the latter considers unreasonable or impracticable, he replies, O yes, I’ll do it – bender; meaning, by the addition of the last word, that, in fact, he will do no such thing.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
[UK]Hotten Dict. of Modern Sl. etc.
[UK]Hotten Sl. Dict.