The term grunch means the sensation a woman has when she was for a time free of conscious awareness of the male gaze and did not feel othered, but is then reminded in some way that she is a woman first and a geek (or colleague, or writer, etc) a distant second. That is, being grunched is the sensation of being firmly and unexpectedly othered.
The word is onomatopoeic, because being grunched includes an unpleasant physical sensation like that of crashing into your own body.
"Grunch" (and the verb "grunched") appear to have been coined by Dorothea Salo in 2002, who also used "grunchy" as an adjective for discussions about sexism.
A woman can feel a grunch happening when she walks into a glass door.