Lying requires knowing the truth and deliberately asserting its opposite, so the liar remains oriented toward facts. The bullshitter, by contrast, is indifferent to whether statements are true or false; the content is selected only to serve a purpose, which is why bullshit is a greater enemy of truth than lies are.
The essence of bullshit is not that a statement is false but that the speaker misrepresents what they are up to. The bullshitter conceals that they are neither trying to report facts nor to deceive about facts; they are simply unconstrained by any concern for how things actually are.
Rather than sorting utterances into true and false, evaluate them by the speaker's underlying relationship to truth. This reveals a category of talk produced without any guidance from reality, which standard true/false sorting cannot detect.
Bullshit proliferates when people are required to talk about matters they do not actually know, as when obligations or expectations push someone to opine on everything. The output is not calibrated to any truth because the speaker has no footing in the subject, only a duty to produce speech.
Frankfurt recounts Fania Pascal telling Wittgenstein, after a tonsil operation, that she felt 'like a dog that has been run over,' to which Wittgenstein replied she did not know what a run-over dog feels like. Frankfurt uses this reaction as a case where a speaker produces words with no regard for fitting reality, exemplifying the carelessness toward truth he calls bullshit.