Lewis Short
(adjective) : impromptus (inpr-), a, um. 2. in-promptus
* Not ready, not quick (rare): infacundior et lingua impromptus,Liv. 7, 4, 5: imprompto jam Arminio ob continua pericla,Tac. A. 2, 21: sermone impromptus Latio,Aus. Idyll. 2, 9.
Charlton T. Lewis, Charles Short, A Latin Dictionary