full of, characterized by, or due to zeal; ardently active, devoted, or diligent.
Origin of zealous
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
filled with or inspired by intense enthusiasm or zeal; ardent; fervent
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
1520s, from Medieval Latin zelosus (source of Italian zeloso, Spanish celoso), from zelus (see zeal). Related: Zealously, zealousness.
Online Etymology Dictionary, © 2010 Douglas Harper