Loser on loser play is a type of declarer's play in contract bridge

Contract bridge

Contract bridge, usually known simply as bridge, is a trick-taking card game using a standard deck of 52 playing cards played by four players in two competing partnerships with partners sitting opposite each other around a small table...

, usually in trump contracts, where the declarer discards a loser card (the one that is bound to be given up anyway) on an opponent's winner, instead of ruffing.

Loser on loser technique can be executed for the following goals:

1. To maintain trump control of the hand,
2. To transfer the ruff to a "safer" suit (e.g. in order to perform a latter crossruff).
3. As part of an avoidance play
   Avoidance play
   In contract bridge, avoidance play is a play technique whereby declarer prevents a particular defender from winning the trick, so as to eschew a dangerous lead from that hand. The dangerous hand is usually the one who is able to finesse through declarer's honors, to give a ruff to the partner or to...
   
    (e.g. the scissors coup
   Scissors coup
   Scissors coup is a type of coup, named so because it cuts communications between defenders, most commonly by discarding a key card from either the declarer's own hand or dummy...
   
   )
4. To rectify the count for a subsequent squeeze play
   Squeeze play (bridge)
   A squeeze play is a type of play late in the hand of contract bridge and other trick-taking game in which the play of a card forces an opponent to discard a card that gives up one or more tricks. The discarded card may be either a winner or a card needed to protect a winner...
   
   .
5. As part of an endplay
   Endplay
   An endplay , in bridge and similar games, is a tactical play where a defender is put on lead at a strategic moment, and then has to make a play that loses one or more tricks. Most commonly the losing play either constitutes a free finesse, or else it gives declarer a ruff and discard...
   
   .

Example

After the auction:

West	North	East	South
1	Dbl	3	4
Pass	Pass	Pass

South plays in a 4-3 spade fit. The defense leads and continues hearts. The declarer has four clubs, two diamonds and four spade tricks in total; however, assuming the most probable 4-2 trump break, if South ruffs the second heart in hand, the opponent with 4 trumps can gain later control of the hand by ruffing one of South's minor winners and cashing the remaining hearts. Thus, South must discard his losing diamonds on 2nd and 3rd heart, allowing the dummy's shorter trumps to ruff the fourth round, or to regain control with a minor-suit winner. After that, South can draw trumps and claim the rest.