Several parts combine into one character.

背 has one of the most etymologically rich derivations in CJK: 北 (now read as "north") on top + 月 (the "flesh" radical, used in body-part characters). But 北 itself originally depicted two people standing back-to-back — a pictograph of literal turning-away. From that "two backs" image, 北 came to mean "north" (because in ancient Chinese ritual orientation, you turned your back to the cold north when facing south to address subordinates). When the body-part meaning of "back" needed its own character, the original two-people image got 月 added as a meat radical, producing 背 = the body part of the back. So 北 and 背 share an ancient root: both depict turning the back, but one became geographical (north) and the other anatomical (back).

The "back" image then radiated outward semantically: back of body → behind / behind one's back → to turn one's back on / betray → to violate. Korean preserves this entire semantic chain: 背景 (baegyeong, "background" — what is behind), 背叛 (baeban, betrayal — turning the back on), 違背 (wibae, violation), 背後 (baehu, the rear / hidden hand behind), 腹背 (bokbae, "belly and back" — front and rear), 背中 (literally back-middle — used in Japanese for "back of body").

Mandarin presents another classic 破音字 (tone-shifting character) with semantic split. bèi (4th tone) covers the noun and the metaphorical extensions: 背景 (bèijǐng, background), 背诵 (bèisòng, "to back-recite" = to memorize / recite by heart — referring to the way memorized text is accessed without the page in front of you, "from behind"). bēi (1st tone) means "to carry on the back / shoulder a burden": 背包 (bēibāo, backpack — literally "back-bag"). Same character (in both traditional and simplified scripts), tone marks meaning.

Japanese on-reading ハイ (hai) — 背景 (haikei, background), 背後 (haigo, the rear / behind the scenes), 違背 (ihai, violation). Multiple kun-readings: せ (se, back / height — note Japanese uses 背 for both "back" and "height," so 背が高い (se ga takai) means "tall" referring to bodily stature), せなか (senaka, the literal back of the body — formed by adding 中 "middle"), そむく (somuku, "to turn one's back" = to defy / betray / disobey). The Japanese phrase 親に背を向ける (oya ni se o mukeru, "to turn one's back on parents") carries the same metaphorical betrayal sense as Korean and Chinese.

Memory aid: 北 (two figures back-to-back) plus 月 (flesh) — the part of the body that turns away.