Arsha prayoga (Sanskrit: आर्षः प्रयोगः) is a common term for such linguistic usages in Sanskrit, which although not correct as per grammatical rules, are still exempted and deemed valid on account of their having been used by some ancient sages (rishis).
Literally, the word Arsha has the following derivation: Its meaning as per Apte's Dictionary is: There are several reasons why deviations from Classical Sanskrit grammar exist in some ancient works.
In practical terms, the word "Arsha Prayoga" has acquired both formal (approving) as well as humorous (disapproving) senses.
In approving sense, it is used to show that the meaning of the word is so important that the speaker's (or writer's) grammatical mistake (or conscious deviation from grammar) does not matter.
While in disapproving sense, it is used in Sanskrit and Hindi, in a humorous way, to point out grammatical ignorance of the speaker (or writer).