Get your dander up
Become agitated or angry.
It is often reported that 'it gets my dandruff up' is one of Samuel Goldwyn's celebrated Goldwynisms. When it comes to having aphorisms that he didn't coin attributed to him, Goldwyn is up there in the big league with Oscar Wilde and Yogi Berra. Whether he made that quip is open to doubt. What is clear is that, like those other supposed Goldwyn coinages - 'statue of limitations' and 'stiff upper chin', the 'dandruff' alternative is repeated now for its comic effect. Dander is an informal term for the skin that is shed from the bodies of animals - dandruff in fact. So, perhaps the Goldwynism may not be all that far from the mark.
If 'get one's dander up' is synonymous with 'get one's dandruff up' we need to explain how dandruff can be 'up' exactly - not an intuitive picture to visualise. The earlier phrase 'get one's hackles up', which has the same meaning as 'get one's dander up', may help. Hackles are the hairs on the back of the neck, especially of foxhounds when excited by an imminent kill, but is also applied more generally. As dandruff may be attached to hair, it is possible that 'get one's dander up' derived as a jokey alternative to the 'hackles' version.
As is often the case with the etymology of old phrases, there's another possible derivation. 'Dander', also means 'ferment', that is, the froth created in the fermentation of yeast in brewing or baking. The word may have been borrowed to form the expression 'get one's dander up' in order to convey the imagery of a frothing or agitated uproar - similar to the associated phrase 'get oneself into a stew'.
There's also a speculation that the phrase was originally 'get one's dandruff up' and that it followed the imagery of a violent and agitated scuffle in which the participant's dandruff might be expected to fly into the air. That one is easily dismissed as the 'dandruff' version post-dates the others. The earliest citation I can find for that version is in The Wisconsin Tribune, April 1853:
"Well, gosh-all Jerusalem, what of it?' now yelled the downeaster, getting his dandruff up."
As we shall see, the 'dander' version is earlier than that.
So, 'get one's dander up' derives from either dander meaning hackles or dander meaning ferment. Both are plausible. Dander did certainly have the meaning of ferment by the time the phrase was coined, as recorded in Sir John Dalrymple's Observations on his Yeast-cake, circa 1796:
"The season for working molasses lasts five months, of which three weeks are lost in making up the dander, that is, the ferment."
Likewise, the 'hackles' meaning was coined at around the same date, as is seen in this entry in the 1786 edition of The Sportsman's Dictionary:
"Some horses have neither scales, dander, or scabs."
The first reference that I can find to 'dander' being 'up' (or raised), that is, being used with the meaning of excitement or annoyance, is from an story in the 1831 edition of The American Comic Annual, by Henry J. Finn, in which a character is teased for his small stature:
" A general roar of laughter brought Timmy on his legs. His dander was raised... straining up to his full height"
It would be nice to provide a clear assurance as to the origin of the phrase but, frankly, both 'fur' and 'ferment' meanings of dander have a reasonable claim and your guess is as good as mine. One thing I can say with certainty is that Samuel Goldwyn didn't coin 'get my dandruff up'.
See other phrases that were coined in the USA.