纯 can be used as an adverb meaning 'purely/simply': 纯是开玩笑 = just joking. Common compounds: 纯洁 (pure/innocent), 纯粹 (sheer/pure), 单纯 (simple-minded/naive).
Left silk radical - simplified 3-stroke form of 糸/絲, a hank of twisted silk. The indexing radical here. Pure unmixed silk was the prototype for 'pure', so the meaning is literal. Same radical heads 红, 线, 织, 纱 - the silk and textile family.
Right component supplies the sound (tún → chún, a regular old-Chinese palatalization across the same dental series). 屯 itself depicts a sprout pushing up through hard ground - fresh and unmixed - which doubles as a faint semantic echo of 'pure'. Same phonetic in 顿, 钝, 沌, 囤.
