Similar to the common insult of referring to someone as a chicken, but taking it even a step further by calling them a chicken that is so particularly needy that they require a bonnet on their head.
The art of pleasuring a man using hands, whilst utilizing the bonnet of a car as a support for the man, without being caught by the owner. See also Auto-Erotica.
"Did you see Emma give that bonnet job earlier?" "I heard Tom got an amazing bonnet job" "There's an arse print on that Corsa, reckon someone's had a bonnet job?"