A phrasal verb is a phrase made up of a verb and a preposition, or a verb and an adverb. A verb is a word that shows action ("throw") or state of being ("is"). A preposition shows the relationship between two things; for example, in the sentence, "The dog is on the couch," the preposition "on" shows the relationship between the dog and the couch. An adverb is a word that modifies any other part of speech but a noun or pronoun ("lovingly").
Phrasal verbs are used commonly in everyday language. For example, we might "call in" to work or tell our friends we will "check out" a movie they recommended. While these words are perfectly acceptable in everyday conversation, and in informal writing, they are generally seen as unacceptable in graduate dissertations and theses. They are viewed as a sort of slang, which is too informal for scholarly writing.
While phrasal verbs are generally not acceptable for scholarly work, some synonymous words and phrases are perfectly acceptable. For example, instead of saying "throw away" you might say "discard." Instead of "put up with" you could say "tolerate." Using these synonyms will not only make your writing more formal, it can also result in stronger, more precise language in your thesis. Editing your work for phrasal verbs can be frustrating, but it will help you create clearer work.
Phrasal verbs are not the only type of word or phrase that should be avoided when writing a dissertation or thesis. Abbreviated forms of words are also generally unacceptable for scholarly writing. For example, use of contractions -- such as "can't" and "don't" -- is not suitable for academic writing. Vague terms such as "stuff" and "things" can also make your work seem weaker. You should also avoid asking questions in your thesis, as your work should be answering questions, not asking them.