恬 is literary and rarely stands alone in modern Chinese. You will see it almost exclusively in compounds: 恬静 (quiet/peaceful), 恬淡 (indifferent to fame), 恬不知耻 (brazen - literally calmly unaware of shame). Note the 心 (heart) radical - the calm is internal.
Left-side heart radical, the upright form of 心 used whenever the heart sits on the left. Carries the meaning: 恬 is an emotional state - calm, tranquil, untroubled. Indexes 恬 in the mood family with 怕 afraid, 怀 yearn, 性 nature, 情 feeling.
Right 舌 supplies the sound - shé drifting to tián through a well-attested old phonetic series shared with 甜 sweet, 括 to bind, 适 suitable. The 'tongue' meaning is not active; 舌 is purely a sound tag here for the soft, settled tián syllable.
