The warm, fond gladness that wells up when something.
The warm, fond gladness that wells up when something — a song, a smell, a place — evokes happy memories of the past; “ah, this takes me back.”
Markedly more positively valenced than English “nostalgic” — joy- and warmth-forward, with little of the melancholy or wistful ache that nostalgia, saudade, or wistfulness carry. It centers on the gladness of recollection in the present moment, and is usually exclaimed spontaneously (“Natsukashii!”) on re-encountering something from one's past, rather than brooded over.
From natsuku “to grow fond of, become attached, keep close”; the kanji 懐 means “bosom, what is held close to the heart.”
From the verb 懐く (natsuku, “to grow fond of, keep close”), held to be a contraction of naretsuku “to become accustomed and cling to.” Kanji 懐 = “bosom.”
A well-attested centuries-long shift from “wanting to keep something close / fondness” toward “fondly recalling the past.” A Japanese-internal development, not an OED matter.
Pop “untranslatable words” articles sometimes overstate that natsukashii has no sad component; in practice it can be gently bittersweet, just far less melancholic than English “nostalgia.”