checkout-line rage
n. Extreme anger caused by a perceived wrongdoing or a lengthy wait in the supermarket checkout line.
Also Seen As
Instead of people treating one other with the kindness, respect and kindred spirit the holidays are supposed to bring, the stress level seems to surge dramatically this time of year.

Road rage. Parking lot rage. Checkout-line rage. Xbox voucher rage.
—Jeff Jardine, “Doesn't anyone recall the reason for the season,” The Modesto Bee, November 28, 2005
The sin of anger has splintered into so many sub-sections (think road rage, supermarket checkout-line rage and the righteous indignation you rightly feel when standing behind someone at a convenience store who is buying lottery tickets) it's hard to pinpoint exactly what it is any longer.
—Lisa Fitterman, “Lust? Sloth? These seven sins are far worse,” The Montreal Gazette, March 21, 2005
1998 (earliest)
News note: A Milwaukee woman, irate because another woman in line before her at the supermarket express checkout counter had more than 10 items, followed her to the parking lot, pulled a pocket knife and slashed away more than half the woman's nose.

There's not just road rage. We also have checkout line rage.
—Howard Kleinberg, “Uncontrolled anger is all the rage these days,” The Atlanta Journal and Constitution, April 15, 1998