媒

Left 女 is the indexing woman radical, a kneeling figure with arms folded, the classic posture for the female-person glyph. Anchors 媒 because the original matchmaker (媒人) was traditionally a woman. Same radical family: 妈 mother, 姐 elder sister, 婚 marriage, 嫁 to wed. The radical signals a person-and-relationship word.

Right 某 supplies the sound, mǒu drifting to méi through regular Old Chinese vowel shift. Same phonetic family: 谋 móu to plot, 煤 méi coal. 某 itself was originally a plum tree, later borrowed for 'certain unspecified person'. Fittingly, the matchmaker introduces 某男某女 'a certain man and a certain woman' to each other.