This is not a word you will find in a dictionary .... yet.
It is a strong insult combining two words 'liberal' and 'retard'.
Politically, conservative / right-wing leaning thinkers consider liberal thinkers to be too soft, relaxed, too easily offended and and allowing society too much expression. They consider this to be a bad thing, stupid or retarded - a very nasty way to describe someone you think is of low intelligence.
Hence, along with being a 'snowflake' (one who is too easily offended by small things) we now have Libtard. (Liberal Retard).