'Unparliamentary language' can hog the limelight, but its close cousin is often more interesting and illuminating.

Parliamentary language is the words you do not hear very much outside Westminster - but which come up surprisingly frequently while listening to politicians.

"Loon" would be one of them.

"Swivel-eyed" another.

There was a great example in the chamber during Prime Minister's Questions yesterday - "crackpot".

Tory MP Sir Peter Tapsell had risen to try to argue that without a second job becoming an MP (salary £67,000) may become the reserve of the rich, "crackpots" and .... those who are "unemployable anywhere else".

On any other week this would have been a cue for MPs on all sides of the House to start pointing at each other.

But following days of serious allegations of 'cash-for-access' his claim attracted only derision.

It also did little to help David Cameron, already under pressure on the issue from Ed Miliband.

Labour was pushing the Prime Minister to sign up to their motion to ban MPs holding outside consultancies or directorships.

The Tory leader rejected that suggestion and tried to embarrass Labour.

Their plans to curb second jobs would still allow MPs to be "trade union officials", he said.

As he spoke Mr Miliband initially looked baffled, briefly conferred with colleagues, and swiftly stood up to agree with the Prime Minister.

Yes, he effectively said, thanks for pointing that out, we'll ban trade union posts too .

'Now will you vote with us?'

Mr Cameron hardly knew where to look.

Labour MPs loved it, clearly thinking they had Mr Cameron on the ropes.

Mr Cameron tried to have the last laugh.

He congratulated a Labour MP on his new post within the party and confidently predicted that in 70 days he could use that position to launch a "root-and-branch review of what went wrong".

Parliamentary language, indeed.