expression, noun
one's younger days; when one was young
A literary or nostalgic expression referring to one's youth, often used in reminiscence. The かりし is a classical adjectival form of 若い.
若かりし頃の思い出を語り合った。
We shared memories of our younger days.
祖父は若かりし頃、船乗りだったそうだ。
I heard that my grandfather was a sailor when he was young.
若い頃 is the modern, everyday equivalent. 若かりし頃 carries a more nostalgic, literary tone due to the classical かりし form.
From 若い (young) in its classical adjectival form 若かりし (attributive of 若かりき) + 頃 (time, period). The phrase is a fixed literary expression.