Cruel like an ungrateful Frenchman (9)

I believe the answer is:

merciless

'cruel' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for merciless that I've seen before include "Uncharitable" , "Totally unforgiving" , "Cruel" , "Showing no compassion" , "Ruthless" .)

I've seen this clue in The Globe and Mail and The Telegraph.

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