taru-adjective, adverb taking the 'to' particle
protuberant; paunchy; bulging
Describes a belly or object that sticks out prominently; often used with 腹 (belly). Literary or somewhat old-fashioned.
彼は便々とした腹をさすった。
He rubbed his protuberant belly.
便々と膨らんだ風船が空に浮かんだ。
A bulging balloon floated in the sky.
adverb taking the 'to' particle
Describes a person or action that is leisurely, idle, or lacking urgency. Often used with verbs like 過ごす (spend time) or 構える (comport oneself). Literary tone.
日曜日は便々と過ごした。
I spent Sunday idly.
彼は便々と構えて、急ごうとしなかった。
He remained idle and made no attempt to hurry.
Positive, relaxed leisure; 便々 can carry a nuance of unproductive idleness or physical bulge.
Reduplication of 便 (ben, 'convenience' or 'excrement'), but the exact semantic shift to 'protuberant' or 'idle' is uncertain. The spelling is conventional.