for many years; anciently
Archaic or highly literary adverb meaning 'for a long time' or 'since ancient times'. Not used in everyday speech; encountered mainly in classical texts or poetic language.
年久しくこの地に住み着いた民の話が伝わっている。
A tale has been passed down of people who settled in this land many years ago.
年久しく語り継がれてきた伝説を、今も村人は覚えている。
The villagers still remember the legend that has been told for many years.
Compound of 年 (toshi, 'year') and 久しく (hisashiku, adverbial form of 久しい, 'long time'). The exact historical derivation is uncertain, but it follows a classical pattern of forming adverbs.