taru-adjective, adverb taking the 'to' particle
Literary word describing a solitary, forlorn state, often with a nuance of helplessness. Used with たる or として.
He stood alone and helpless in the wilderness.
He was washed ashore on a solitary, isolated island.
The kanji 孑 means 'alone' or 'mosquito larva', and 然 is a suffix forming adjectives/adverbs. The exact derivation is uncertain, but the compound is a literary Chinese borrowing.