The Rice University Neologisms Database

Etymology : Variation of “L,” a clipping that stands for “loss.” “L” and its counterpart “W” for “win” are usually used to describe something that doesn’t typically result in a clear “win” or “loss”, such as a test where there are numerical grades rather than passes or fails, but still has a desirable or undesirable outcome. Half L uses the idea that the result of a situation can be quantified into degrees of winning or losing, with this particular situation being only a partial, or half, loss since it wasn’t the worst outcome but still wasn’t favorable.