lemon, adj.² — Green’s Dictionary of Slang

lemon adj.²

[rhy. sl.; lemon squash = flash adj. (1a)]

1. ostentatious.

2000		J.J. Connolly Layer Cake 65: In this end of town it’s all young babes with tit-jobs, messers, chancers and hustlers, double-lemon rag-trade guys, lemon meaning flash.
2004		(con. 1980) N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 246: Suddenly the Kent firm started getting a bit lemon [...] I didn’t like being patronized by this flash monkey.

2. (UK Und.) aggressive, ‘sour’.

2007

N. ‘Razor’ Smith Raiders 183: He started to get a bit lemon and said that my refusal to negotiate would have ‘consequences’.