excessive, too, very-usually of objectionable things, to err, to mistake, changeable
忒 centers on going beyond a limit, usually in a negative way: being excessive, too much, or erring. The idea of changeability connects to shifting past a proper boundary.
忒 combines 弋 (a stake or arrow) with 心 (heart/mind), suggesting a heart that strays or goes too far. The exact historical development is uncertain.
The outer 弋 looks like a stake driven into the ground, and the inner 心 is the heart. Picture a heart staked too far out, becoming excessive and prone to error.
For トク, imagine a person who talks too much: 'talk' -> トク, and their excessive chatter is objectionable.