na-adjective
Abbreviation of オフトピック, used in casual online conversation to indicate a comment or post is off-topic.
See also: オフトピック
その話はオフトピだから、あとでDMで話そう。
That topic is off-topic, so let's talk about it later via DM.
ちょっとオフトピだけど、昨日のアニメ見た?
This is a bit off-topic, but did you watch yesterday's anime?