The phrase 'off subject' is correct and usable in written English.
You can use it when you want to indicate that a conversation or discussion is shifting away from the original topic at hand. For example, "We were discussing the curriculum changes, but then the conversation went off subject and we started talking about the new lunch menu."
QUESTION FROM WINN CARROLL: A bit off subject, but, Adam, I just got done reading "From Paris to the Moon" and loved it.
In danger of straying slightly off subject here but figured the Leicester fairytale may be wearing slightly thin on (the more miserable among) you.
(Conversation on the sports station, which The Philadelphia Inquirer has called the antidote to National Public Radio, often wanders off subject).
QUESTION FROM MORGAN: Slightly off subject, has there been development in the Chinese reaction to the Occupy Wall St. protests around the US? EVAN OSNOS: Yes, it has been embraced — actually, make that bear-hugged — by the state media, which is having a lot of fun with the idea of protests in NYC.
Another source of doubt is the thought that the Rawlsian difference principle, the identification of distributive justice with maximizing the prospects of the very worst off (subject to the FEO constraint), is too extreme in the priority it assigns to achieving gains for the very worst off.
Many rejected Aristotle's claim that some human beings were by nature less than wholly rational and so were best off subject to the mastery of others, yet even so they did not challenge the existence of the institution of slavery itself, although they might entertain doubts about its legitimacy.
Sven was also right to warn the press off subjecting Wayne Rooney to a Beckham-style hounding following his sending off.
Class sizes have gone up, school trips are off, subjects have been scrapped and pastoral care has been axed.
Miller rattled off subjects he would like to feature, from an overnight security guard at the Detroit Institute of Arts to someone who drives a snowplow.
Next, a performance merit figure is proposed as the main research contribution, which addresses the power consumption and signal quality trade-off subject.
This illustrates the biggest source of trouble — intervening phrases that throw us off track, most often tricking us into a plural verb even though the far-off subject is singular.
