This noble lord is listed in some editions of the libretto but not others. (He is a bass singer who is used in the madrigal in place of Pish-Tush if the latter cannot hit the low notes.) The expression is a bob-tailed version of "Go to ______" (fill in to suit your mood). Brewer (54) says it is an "exclamation, often of impatience or reproof, or as an exhortation like 'Come'." Shakespeare used it in Julius Caesar. More to the point, Gilbert has both Pooh-Bah and Katisha saying it in The Mikado. Prestige (245) says it is a mild reproof similar to "pish" or "tush."