An idiot end is a low straight in a game with shared cards. It is called idiotic because it is so vulnerable to higher straights. In particular, the idiot end occurs when four straight cards on the board and the player holds the low card to complete the straight. Only an idiot would believe himself to be ahead with such a hand if he encountered great resistance.
Example (Texas Hold'em):
The player has a straight with the ace, but any six will beat him. He has an idiot end.