The answer to 'grout filthy' is that grout can become discolored and dirty over time due to a variety of factors, including spills, mold, mildew, and general wear and tear. Symptoms of dirty grout include stained, discolored, or cracked grout lines that can make your tile floors or walls look old and unkempt.
In many cases, you can clean dirty grout yourself using a grout brush, grout cleaner, and some elbow grease. However, for heavily soiled or damaged grout, it may be best to call in a professional tile and grout cleaning service. They have the right tools and expertise to thoroughly clean and restore your grout to like-new condition.