slow burn

Example: “When she read the email, you could see her doing a slow burn.”

Origin: American slang first documented in 1938, closely associated with the film comedian Edgar Kennedy, who built much of his signature comic persona around a specific physical technique: visibly, gradually escalating from calm to quietly seething to fully explosive rage, stretched out across many long seconds for maximum comic tension rather than snapping into anger all at once. "Burn" itself had already carried the figurative sense of "become furious" well before this specific idiom — "burn up" for boiling over with anger is older still — so "slow burn" simply specified the pace: not a sudden flare, but a steadily building, visibly mounting one.