stature, height, back, behind, disobey, defy, go back on, rebel
背 centers on the back side of something: the physical back of a body, the rear of an object, or the figurative idea of turning one's back on something, which leads to disobeying, defying, or rebelling. The stature/height sense is a related extension from the back's role in posture.
背 combines 北 (originally depicting two people back-to-back, suggesting 'back') with 月 (flesh/body part), reinforcing the idea of the back of the body. The modern form preserves this structure clearly.
The top 北 looks like two people standing back-to-back, and the bottom 月 is flesh. Together they show the fleshy back of a person. Turning your back on someone can mean disobeying or rebelling.
For ハイ, imagine someone turning their back and waving 'Hi!' sarcastically while walking away: Hi -> ハイ, and the back is turned in defiance.
background; scenery; backdrop; setting
back (of the body)
height; stature
back
to carry on one's back
back; rear
rear; back; reverse
spine; line of the backbone
number on the back of a player's uniform; uniform number; shirt number; squad number
spine; backbone; spinal column
corruption; immorality; lapse from virtue; fall from virtue
to turn (one's face) away; to avert (one's eyes)
bent back; hunchback; stoop
to run counter to; to go against; to disobey; to infringe
standing on tiptoe and stretching one's back to make oneself taller; stretching oneself
business suit
stature; height
back (of a chair); backrest
(one's) back
back-to-back; facing in opposite directions