an informal, simplified spelling of low1, used especially in labeling or advertising commercial products: lo calorie.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
look! see! (now often in the phrase lo and behold)
Word Origin for lo
Old English lā
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
early 13c., from Old English la, exclamation of surprise, grief, or joy; influenced in Middle English by lo!, short for lok "look!" imperative of loken "to look" (see look (v.)). Expression lo and behold attested by 1779.
Online Etymology Dictionary, © 2010 Douglas Harper