• 第一だいいち — first / number one
• 第三者だいさんしゃ — third party
• 第六感だいろっかん — sixth sense
• 次第しだい — order / as soon as / depending on
• 落第らくだい — failed grade

第 combines the bamboo radical ⺮ (たけかんむり) with the element 弟 as a phonetic. The character marks sequence or rank, which is why it’s placed before numerals to form ordinals like 第一 and 第二.