Generally, from what I've gleaned, if a left wing person is using Liberal in a derogatory way, then they are referencing ideologies that prioritize personal liberties and property over greater good. A liberal in that sense wouldn't necessarily like socialist or communist policies that may have more taxes, regulations, or limitations to their personal life. I'm not claiming to be an expert, mind you.

The example I can think of (as I understand it), is say let's use single payer Healthcare. Government takes over, taxes people more, and limits the options and choices of doctors. While in general a greater good, some people who prefer personal freedoms may be upset something that used to be covered or a personal doctor they liked doesn't work under the new setup (like certain homeopathic or chiropractic not being covered as real medicine).

A liberal in this case may value their personal choices as more important. NIMBYs also count; like those who don't want low income housing near their own homes. They may claim to want to support the poor and homeless, but object if it affects their housing values or makes them uncomfortable.

I'm not sure on this next, but I'll add it anyway. The US in particular but other "liberal" countries as well have a history of exploiting weaker/less developed countries for our own comfort. Sure we all say we hate banana Republics and what the US did for those to work, but how many of us would willingly give up comforts we gain from that exploitation?