第 sets 弟 (which itself drew cord wound in sequence around a stake) under bamboo ⺮: bamboo book-slips bound in order. Sequence was physical — the order your bamboo pages were strung.
第 turns numbers into rankings: 第一 (first — also 'the best': 世界第一, world number one), 第二次 (the second time), 第三天 (the third day). Imperial exams made 及第 (passing) the word for making it; failing was 落第. Modern China still runs on 第一名 (first place) — from kindergarten to Olympic podiums. Every ordinal you'll ever need: just add 第.
Bamboo pages ⺮ strung in order — the -TH in every ordinal.
安全第一 'safety first' — the construction-site slogan — uses the same 第一 as Olympic gold. Priorities, ranked.