trewd-
“to press, to push, to thrust”push, thrust, squeeze
Root for pressing/pushing, yielding English threat, thrust, Latin trudere (to push).
Discussion
The PIE root *trewd- (to press, to push, to thrust, to squeeze) produced the Latin vocabulary of forceful insertion and the English vocabulary of threat — all unified by the image of pushing something where it doesn't naturally belong.
Latin trūdere (to thrust, to push, to shove) continues the root and gave English an extensive family built on prefixed compounds: intrude (in-trūdere, to thrust oneself in — to push into a space where one is not wanted), extrude (ex-trūdere, to thrust out — to push material through a shaped opening), protrude (prō-trūdere, to thrust forward — to stick out), obtrude (ob-trūdere, to thrust against — to force oneself on others), and abstruse (abs-trūdere, to thrust away — pushed so far from view that it becomes hard to understand).
The semantic development from physical pushing to intellectual obscurity (abstruse) is characteristic of Latin abstract vocabulary: what has been pushed away is hard to see, hence hard to understand.
English threat (OE þrēat, "a crowd, a pressing, an oppression" — from PGmc *þrautaz) continues the root through Germanic with the specific sense of pressure that menaces. The original OE meaning was not a verbal warning but physical pressure — a throng pressing against you. The modern sense of "a declaration of harmful intent" developed in Middle English as the concept of pressure shifted from physical to psychological.
The word thrust itself (from Old Norse þrysta, "to press, to squeeze") may be from the same root through a different formation.
The root connects to *steh₂- (to stand) through the concept of resistance — what is thrust pushes against what stands firm — and to *bʰreg- (to break) through the concept of force exceeding resistance.
Notes
Pokorny 1095. English threat, thrust, intrude, extrude, protrude.