The blue jelly, j, is a monster in NetHack. It is sessile, making it useful for boulder forts, stashes, and the like. Blue jellies cannot attack directly, but their passive attack, which causes you to be "suddenly very cold", can be deadly to inexperienced players. Blue jellies are blind.
If you attack a blue jelly without killing it, and you do not have cold resistance, it will gain HP equal to half the damage it inflicts on you. Attacking it, even if you miss, may cause the blue jelly to divide in two (“The blue jelly multiplies from your heat!”).
Avoid attacking in melee unless you have cold resistance, as blue jellies are much tougher than might be expected. At their base level, they do (4+1)d6 damage, which gives a range of 5–30, and they can be generated at much higher levels. Ranged attacks are preferable, but make sure you have enough ranged weapons to finish the job—you won't be able to retrieve that lovely stack of daggers until the jelly is dead.
If you lack cold resistance and have sufficient HP, you may also be able to farm blue jellies for food or its two intrinsics, though you're likely to spend more nutrition splitting a jelly than you will get from the corpse.