conjecture
揣 centers on forming an opinion without full evidence: guessing, estimating, or conjecturing. The given meaning "conjecture" captures this act of tentative judgment.
揣 combines 扌 (hand) with 耑, which likely contributes the sound and may relate to measuring or estimating by hand. The exact historical development is uncertain.
The left side is a hand 扌, and the right side 耑 looks like a sprout pushing through a boundary. Picture a hand trying to gauge the height of a sprout without measuring—just a rough conjecture.
For シ, imagine a hand making a guess and the answer is a simple "she"—she -> シ, as if the hand is pointing at someone and conjecturing who it is.