Literally, ‘not on his/her plate’, the ‘assiette’ in this case originated in the 13th century from the latin assedita, which meant ‘sit’ and the expression at the time alluded to taxes.
I just wanted you to know I do my research on this stuff sometimes.
Anyway, fast forward to Stage 8 of Paris-Nice and we find out that Ineos’ Martinez is definitely ‘pas dans son assiette’ today, as he gets dropped at the foot of the Col d’Eze.

In non-13th century French, this means that he’s not feeling it / out of sorts. Pogacar, however, was definitely on his plate.
pogi was looking good I can say