expression
privately; just family/close ones; with no outsiders
Used when a group, especially family or close friends, spends time together without any outsiders present. Often implies a cozy, intimate atmosphere.
今夜は家族水入らずで食事を楽しんだ。
Tonight we enjoyed a meal with just the family.
久しぶりに親子水入らずで温泉に行った。
We went to a hot spring, just parent and child, for the first time in a while.
内輪 means 'inside the group' or 'among ourselves' and can be used more broadly for any in-group, while 水入らずで specifically emphasizes the absence of outsiders in a warm, intimate setting.
Literally 'without water mixed in', from 水 (water) + 入らず (negative of 入る, to enter) + で (particle). The metaphor suggests an undiluted, pure gathering of close relations, like undiluted liquid.