"Crass" suggests something vulgar. "Careless" suggests neglect. Check your dictionary again, and tell us if you're still confused. These are very different words.

stupid, dull, dense, crass, dumb mean lacking in power to absorb ideas or impressions. ------------------- stupid implies a slow-witted or dazed state of mind that may be either congenital or temporary <stupid students just keeping the seats warm> <stupid with drink>. dull suggests a slow or sluggish mind such as results from disease, depression, or shock <monotonous work that leaves the mind dull>. dense implies a thickheaded imperviousness to ideas <too dense to take a hint>. crass suggests a grossness of mind precluding discrimination or delicacy <a crass, materialistic people>. dumb applies to an exasperating obtuseness or lack of comprehension <too dumb to figure out what's going on>. -------------------------- see? There's always a difference

Remember that there are "NO" two words in English that mean exactly the same.