In British English the word has a variety of profane implications, inviting childish sniggers from grown men: http://www.urbandictionary.com/define.php?term=knob

The following are quoted extracts from Roger's Profanisaurus:

"knob 1. n. Penis; prick; John Thomas. 2. v. To shag. As in “I knobbed the arse off her”. knob cheese n. 1. Smeg-ma. 2. Foreskin feta found underneath Kojak’s roll-neck (qv) and around the banjo (qv) or cheese ridge. Also knob Stilton, knob yoghurt, Helmetdale (qv). knob chopper n. Precariously balanced lavatory seat which falls down while you’re having a piss and therefore must be held up manually. knob head . knob end n. Fuck wit; thick head; oaf. knob jockey n. Someone who sits on a door handle whilst the door is in motion for sexual gratification. knob shiner n. Someone who is always prepared to polish a pink oboe (qv)."

Source: http://en.wikipedia.org/wiki/Rogers_Profanisaurus