Can you cast the grease spell on an item (like a weapon)?
In 5th Edition, this is not possible, per the description of the Grease spell in 5e:
Slick grease covers the ground in a 10-foot square centered on a point within range and turns it into difficult terrain for the Duration.
When the grease appears, each creature standing in its area must succeed on a Dexterity saving throw or fall prone. A creature that enters the area or ends its turn there must also succeed on a Dexterity saving throw or fall prone.
You may be remembering that older editions gave Grease the ability to target an item. In Pathfinder and in 3.5 for instance. Here's the description of the Grease spell in PF:
The spell can also be used to create a greasy coating on an item. Material objects not in use are always affected by this spell, while an object wielded or employed by a creature requires its bearer to make a Reflex saving throw to avoid the effect. If the initial saving throw fails, the creature immediately drops the item. A saving throw must be made in each round that the creature attempts to pick up or use the greased item. A creature wearing greased armor or clothing gains a +10 circumstance bonus on Escape Artist checks and combat maneuver checks made to escape a grapple, and to their CMD to avoid being grappled.