thick on the subject

[James Harbeck]

"Thick," particularly in British English (but evidently also in
Maureen Dowd's dialect), means "stupid" ("Are you thick?!") -- from
"thick-headed," I believe. To say someone's "thick on the subject"
thus means that when it comes to the topic at hand they are
noticeably ignorant and perhaps difficult to instruct. So, as I read
it, it's really the one-word idiom "thick" plus the reasonably common
phrase "on the subject" (= about the subject, as regards to the
topic, etc.).

James Harbeck.

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