Green’s Dictionary of Slang

1. anything large, or impressive of its type.

1989		Guardian 10 Aug. n.p.: In fact he was driving well and at the 15th, where you are required to let out the braces a bit, produced his No. 1 stonker.
1999		Indep. on Sun. Culture 13 June 6: That gloriously bulbous, outrageous stonker of a nose.
2001		OnLine Dict. of Playground Sl. 🌐 stonker n. [...] (2) excellent, really good, impressive ‘The party was great – a real stonker!’.

2. (also stonk, stonk-on) an erection.

1987		P. Yates Sex with P. Yates v 66: When a man gets very excited, he gets an erection, a.k.a. a hard-on, stonker, etc.
2001		letter to World Wide Words 🌐 I seem to remember the word ‘stonk’ was used in my youth (Leicester, East Midlands) to refer to a penile erection – ‘to have a stonk on’ meaning to be in a tumescent state.