Mjǫllnir is the name of Þórr's hammer. The name for the hammer derives from Proto-Norse *melluniaR with uncertain etymology. Theories include:
1) Related to Old Slavic mlunuji, Russian molnija = 'lightning' (either borrowed from there or else from an early stage) which would allow an interpretation of 'the one who makes lightning' [1]
2) Related to Old Norse mjǫll = 'new snow' (see also MJALL), Icelandic mjalli = 'white colour'. As such it would mean 'the shining lightning weapon' [1]
3) In earlier scholarship it had been connected with Gothic malwjan and Old Norse mala = 'to grind' and interpreted as 'the grinder' [1]
See also Mjöllnir