The word meant something the first time I saw it, sometime in season 3. I understood very clearly what the person saying it meant, and the reason they said it.
Now, when someone gets first blood due to an early gank, we're lucky if the victim doesn't get on [All] to call everyone tryhard.
In another recent thread, I was told that building a certain champion to maximize damage to enemies is tryharding, whereas building that certain champion in order to take game objectives when no one is looking is not tryharding. (I'm not saying anything about one build being better than the other).
I used to mentally apply the word tryhard it to situations where someone was actually trying hard to do something very stupid; for instance, if a jungler and an ADC attempted the same gank setup over and over, failing miserably every time after the first... that would be worthy of the word tryhard.
I don't use this word anymore, since its new meaning is clearly "I'm losing fights and that's not fair."