Not 'how tf (the fuck)", an expression of shock, but "How? tf? (transformation)?", a question of the precise mechanism of this transgender breastfeeding scene.

Presumably, they assumed the person in question was a trans woman, (which may not be the case, it may be a trans man, I do not know), and wondered if this was a case of them being physically transformed into a female with functional milk glands, or just handwaved. (Because, while trans women can lactate in real life, it usually requires an experimental drug regimen.)