Snake in the grass

phr.

Heard in Carlow and Kildare

A deceitful, untrustworthy, or excessively sly individual.

In use

I wouldn't trust him an inch, the absolute snake in the grass.

Posh English

I should not repose any confidence in him whatsoever, displaying as he does the manners of a most deceitful fellow.

Same phrase, other counties