A squinting modifier is one that could modify either the word(s) before it or after it (i.e., it is ambiguously positioned in the sentence.) It is a "misplaced modifier."
Cycling up hills quickly strengthens your quadriceps.
(In this example, the word "quickly" is a squinting modifier. It is not clear whether "quickly" pertains to "Cycling up hills" or "strengthens".)
"He was a hero at his last police station. He once shot a robber with a Kalashnikov." "Great, where did he get that?" "No, the robber had the Kalashnikov."
(In this example, the phrase"with a Kalashnikov" (a type of assault rifle) is a squinting modifier. It is not clear whether it pertains to "he " or "the robber.") (This is taken from the film "Hot Fuzz.")
Glossary of grammatical terms