Unbecoming of one's position - beneath one's dignity.
This derives from the Latin infra dignitatem, literally - 'beneath (one's) dignity'. It is first recorded by William Hazlitt in Table talk; or, original essays on men and manners, 1822:
"Among other things, the learned languages are a ready passport to this sort of unmeaning, unanalysed reputation. They presently lift a man up among the celestial constellations, the signs of the zodiac (as it were) and third heaven of inspiration, from whence he looks down on those who are toiling on in this lower sphere, and earning their bread by the sweat of their brain, at leisure and in scorn. If the graduates in this way condescend to express their thoughts in English, it is understood to be infra dignitatem..."
The first person to put th shortened infra dig. version into print was Sir Walter Scott. He uses it in his 1825 novel Redgauntlet:
"It would be infra dig. in the Provost of this most flourishing and loyal town to associate with Redgauntlet."
It is now more commonly written without the full stop. Even most of those who realize it is an abbreviation now consider it to be well-enough established not to require it, as amp - short for ampere - is now accepted without a full stop.
What is beneath one's dignity is obviously a matter of judgment. The group most often associated with the term are the British upper classes, although they might now consider it infra dig ever to use it.
See also - Latin Phrases in English.
See also - phrases coined by Sir Walter Scott.