Literally: empty – birth / arbitrary – delusion – explanation
Alternately: Groundless remarks. False statements. Talking nonsense; making irresponsible comments; shooting off at the mouth.
Notes: This is another two-two-character-compound-compound: 虚誕 refers to not just falsehoods but exaggerated falsehoods, while 妄説 refers to unsupported assertions or false claims. The latter term can be pronounced bousetsu when used on its own, but not in this four-character compound.
This usage of 誕 is a bit of a revelation to me, actually – it’s almost only ever used in modern Japanese as part of the term 誕生 (tanjou), “birth.” Apparently the combination of 言, “word,” and 延, “extend” originally meant “words that extend beyond reality” – i.e. a falsehood. It can also mean “(speaking of) things as one wants them to be (rather than how they really are),” and at this point we have to be careful because the folk etymologies bridging the gap to “birth” practically jump at you… but assuming them to be true without in-depth research would just be another case of 虚誕妄説!