Is it okay to use muriatic acid undiluted?

yes if it won't hurt what your cleaning. it will mildly irritate the skin but it won't injure you It depends on what you are using it on - to clean concrete or masonry residue. Wet the surface with water first, then apply and brush on. Muriatic acid should be diluted at least 5 to 1. The less dilution the more the vapor will create a breath taking green gas. So stay upwind and wear proper gloves and eye protection. If is is undiluted it will leave a residue that will need to be nuetralized. Copious amounts of water -flushing works, but there are commercially available cleaners at Home Depot, for example that will do a good job.