1
general::
1. Below cost, as in The store was doing so badly that it was selling merchandise at a loss. 2. Puzzled, perplexed, in a state of uncertainty, as in When his letters were returned unopened, John was at a loss as to what to do next. This usage was originally applied to hounds who had lost the scent or track of their prey. [Mid-1600s] 3. at a loss for words. Unable or uncertain as to what to say. For example, Father's tirade left us all at a loss for words. [Late 1600s]
American Heritage Idioms
2
general::
Phrase(s): at a loss (for words)
Fig. unable to speak; speechless or befuddled. • I was so surprised that I was at a loss for words. • Tom was terribly confused—really at a loss.
McGrawhill's American Idioms And Phrasal Verbs