What's the meaning of troop on in this comment?
Troop on, brotato. Get well soon.
The comment was made in response to someone who's suddenly gotten really sick and has to stay in the hospital. Is this some kind of slang?
Common senses of the verb troop seem not-applicable here, where the sense in use might instead be a back-formation from a well-known sense of trooper, “One who endures adversity or hardship with an attitude of stoicism and persistence”; or it may be, as FumbleFingers suggests, a variant of soldier on, carry on, march on. That is, in this context troop on means “endure this adversity without letting it overcome you” .
While not used often, I imagine it is similar to "soldier on", which means
Other more common phrases are "keep your chin up" or "hang in there".
I suppose the British might say something like "keep a stiff upper lip" but I don't know how current that is.
(In French, it's « Bon Courage ».)