to a small degree or extent
"his arguments were somewhat self-contradictory"
"the children argued because one slice of cake was slightly larger than the other"
Slenderly; delicately.
"The dancer moved slightly across the stage, her toes barely grazing the floorboards as she extended a single arm toward the audience."
In plain English: Slightly means just a little bit or not very much at all.
"The coffee was only slightly warm, so I waited for it to cool down before drinking."
The word slightly is formed by adding the suffix -ly to the adjective slight. It entered English as a straightforward grammatical construction without any shift in its original meaning.