As cool as a cucumber

Cool here means imperturbable rather than low in temperature, although the simile is formed by alluding to the coldness of cucumbers which, being mostly water, are cool to the touch.

‘As cool as a cucumber’ is first recorded in John Gay’s Poems, New Song on New Similes, 1732:

My passion is as mustard strong;
I sit all sober sad;
Drunk as a piper all day long,
Or like a March-hare mad.
Round as a hoop the bumpers flow;
I drink, yet can’t forget her;
For, though as drunk as David’s sow,
I love her still the better.
Pert as a pear-monger I’d be,
If Molly were but kind;
Cool as a cucumber could see
The rest of womankind.

Gay used several similes in his poem that we still use today, but others, like ‘pert as a pear-monger’ have gone entirely defunct. Indeed it’s some time since the selling of pears was a full-time occupation.

See other ‘as x as y similes‘.