Tartle is a word that has been painfully misconstrued to the point that you should never use it. Unfortunately, it's one of those poor words that just keeps getting picked on...
In modern times, the word 'tartle' appears almost entirely in clever lists of words titled, inanely, as 20 awesomely untranslatable words from around the world, or even worse, Foreign Words to Which English has no Answer. This is painful both because these sources uniformly give a simple and direct translation without any hint of irony, and because tartle is almost certainly an English word.
Tartle - From Scottish, the act of hesitating while introducing someone because you've forgotten their name.
Well, no. Tartle has a number of meanings, all of which are more general than "to hesitate because of a forgotten name", and none of them exciting enough to get on a list of untranslatable words. However, as it happens, tartle is a contronym, as it means to both to recognize and to not recognize.
1. v. To recognize; to observe."He never tartled me."
2. v. To boggle; to raise scruples; to hesitate.
3. v. (Especially in the idiomatic phrase "To tartle at one") to be uncertain as to a person's or thing's identity.
And as to the anti-factoid that tartle is a foreign word... Well, aside from the obvious fact that Scottish is, by definition, the dialect of English used in Scotland, it is most likely that tartle doesn't have a Gaelic origin. It is most likely from the same root as is 'startle' (and in fact, is most likely the same word), from the Old English sterten meaning 'start'. This etymology is most closely tied to the second definition of tartle, as startle came to be used in that sense in the late 1500s. The other meanings appear to be idiomatic usages unique to the Scottish dialect.
References:
An Etymological Dictionary of the Scottish Language by John Jamieson; 1818
Jamieson's Dictionary of the Scottish Language By John Jamieson, John Johnstone, and John Longmuir; W. P. Nimmo, Hay, & Mitchell; 1895
The Free Dictionary: Tartle