Not with Word Online. Unlike the desktop version of Word, Word Online doesn’t have a built-in thesaurus for looking up synonyms.
If you have Word for the desktop, click Edit Document > Edit in Word, or click Open in Word to open your document.
Then follow the instructions in Look up words in the thesaurus.
Tip: Word Online works seamlessly with Word and other Office desktop programs, and we update it based on input like yours. For the latest feature announcements, visit the Office Web Apps blog.