From The Collaborative International Dictionary of English v.0.48 :
buttoned-down \buttoned-down\ adj. conservatively formal and businesslike in dress and manner. Syn: button-down, conservative. [WordNet 1.5] a colorful character in the buttoned-down, dull-gray world of business. --NewsweekFrom WordNet (r) 3.0 (2006) :
buttoned-down adj 1: unimaginatively conventional; "a colorful character in the buttoned-down, dull-grey world of business"- Newsweek [syn: button-down, buttoned-down, conservative]