noun
face lost in pleasure; ecstatic trance face
Slang, often vulgar or sexual, describing a slack, blissed-out facial expression. Related to アヘ顔 but generally softer and less extreme.
See also: とろん · アヘ顔 (アヘがお)
彼女はマッサージ中にトロ顔になっていた。
She had a blissed-out face during the massage.
あの漫画のキャラはいつもトロ顔だ。
That manga character always has a face lost in pleasure.
とろん is an adverb describing a dazed or dreamy state, often modifying する or した目. トロ顔 is the noun for the resulting facial expression.
Compound of トロ (from とろん, meaning dazed or dreamy) and 顔 (face). The exact derivation is uncertain, but it likely emerged in otaku or internet slang.