Technically, a receptacle used to collect semen (usually, but not exclusively, human) excreted through acts of either auto-erotica or independent stimulation.

More commonly, a wank sock.

Colloquially employed as an term of reference when referring to the producer of a certain Saturday early evening broadcast on BBC Radio 4. Whose name rhymes with hymen. Which is apt as he is a bit of a cunt.
That Simon, he's a right Gerrard's sock.

Often truncated to -

That Simon, what a sock.

Can be used in combination with any variety of terms -

What a fucking sock.

Or even as an adjective -

What a socking tool.
by Mrs Clancy February 14, 2005
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