To go under s.t. with an instrument in order to remove it (as weeds, head lice, etc.); to scoop up s.t.; to scrape s.t.
The -un imp. seems to focus more on the action of going under s.t. definite. The -an imp. focuses on the definite object being removed because it is of no use.
When you weed, uproot them so it will be a long time before they grow again.
Scoop up that dog's feces as it stinks badly.