Dislcaimer: The interpretations of the glyph forms and OC reconstructions are my personal opinion. Professional opinions state that this character is still open for interpretation.
I guess you can view「克」(Baxter-Sagart OC: /*kʰˤək/, to achieve victory over/subdue/defeat an opponent in a military confrontation), as being a merger between semantic「由」(battle helmet, now written as「冑」) and reduced phonetic「革」(/*kˤrək/). The graphically reduced「革」looks like「⿱廿尸」or「⿱口尸」(picture of animal skin and horns).
Specifically, the bottom part of「由」is merged with the top part of the reduced「革」.