A person who likes to correct other people’s grammar and judge them by it.
(this
definition taken from a book review on bookriot.com)
An obvious portmanteau of "grammar" AND "commando."
Someone
like Sheldon, of The Big
Bang Theory, who is constantly on the alert for
any opportunity to find a flaw in
one's presentation of data/information and then
bombastically inform
one (and anyone else within earshot) of that flaw,
right down to the tiniest misplaced comma or improper verb tense.
Being an internal monologue grammando while reading or watching TV is
fine, so long
as it's just for your own amusement/improvement, but engaging your outer grammando
during an author's book reading is likely to see you educated with a LART
(Luser Attitude Readjustment Tool) by that author one of his/her/its fans.