backup boyfriend; man kept as a backup marriage partner
Colloquial term for a man a woman keeps as a romantic backup, often while pursuing other relationships. The 君 suffix adds a casual, slightly dismissive tone.
See also: キープ
She has a main boyfriend, but apparently she also has a few backup boyfriends.
From キープ (keep) + 君 (kun, casual suffix for males). Likely coined in modern colloquial Japanese to describe a backup romantic partner.