the only way; the sole course
Used to express that there is no alternative; often appears in phrases like 一途をたどる (to follow the only path) or 一途しかない (there is no other way).
He continued walking the only path to success.
In this situation, there is no choice but to retreat.
continuous decline; on the wane
Compound of 一 (one) and 途 (way, path). The exact historical derivation is uncertain, but the meaning 'the only way' is transparent from the kanji.