People seem confused by it but isn't it just a play on words with/combination of 'I think therefore I am' and sexual slang that often takes the form she blank til I blank?

Cogito my ergo until I sum

I can't tell if you had a stroke or I did.

a Cartesian courtesan

Honestly, if it said “til” and not “till,” I’d want one.

I can’t tell if there is a sexual innuendo here that is intended.

She could from face that

Might be one of those shirts sold in a foreign land where not the designer, the buyer, nor the readers will actually know english.

Just like those white guys with tattoos in Mandarin that read "I Like Corn".

She on my till

Think therefore I am