noun
Archaic female term of endearment for a husband, lover, or close male friend. Not used in modern Japanese outside of historical or literary contexts.
「背子よ、そなたを思う心は変わらぬ」と古い文に書かれている。
In an old letter it is written, 'My love, my feelings for you will never change.'
noun
my friend; comrade
Archaic male term for a close male friend. Like sense 1, it is obsolete in modern speech and appears only in historical or literary works.
武士たちは互いを「背子」と呼び合ったと伝えられる。
It is said that warriors called each other 'seko' (my friend).
The exact derivation is uncertain. The kanji 背 (back, stature) and 子 (child) are conventionally associated with the meaning, but the historical origin of the reading せこ is unclear.