taru-adjective, adverb taking the 'to' particle
tipsy and cheerful; merry with drink
Literary or classical expression describing a pleasant, slightly drunk state. Often used in the form 醺然として or 醺然たる. Rare in everyday speech.
He was tipsy and cheerful from the sake.
In a merry drunken mood, he began to sing.
The kanji 醺 means 'drunk' or 'tipsy', and 然 is a suffix forming adjectives/adverbs. The exact historical derivation is uncertain, but the word follows a classical Chinese pattern for descriptive compounds.