Arrrh, me hearies! Today be Talk Like A Pirate Day, on which day I be wantin' to answer a question that gentlemen o' fortune all o'er the briny blue ha' been askin' theyselves: what be the part of speech of avast? Aran, me wee, pirate-mad son, runs around the house tellin' everyone to avast. (For him, every day be Talk Like A Pirate Day, arrh.) Aran uses avast as an ordinary verb, the scallywag. Out of his mouth ha' come , "You must avast," "I avasted," and "You're not avasting." Now 'tis normal for children and other sprogs to over-generalize the words they be hearin', but no buccaneer tha' ha' reached the age to go to sea be usin' the word so. 'Tis not a normal verb, by thunder, so what be it? The pack of squiffies that be writin' dictionaries give no help, the scurvy dogs! American Heritage calls it an interjection. Merriam-Webster says 'tis an verb imperative, and The OED calls it a phrase. A phrase? Well, avast be derived from the Dutch phrase houd vast (hold fast), but 'tis peculiar to call it a phrase today. 'Tis also strange to call it an interjection. Interjections be words like aye or ahoy that have walked the plank. They be adrift, with no syntactic connection to the rest of the sentence. But avast be part of the crew in a sentence. It can take a complement, as in avast hauling. Merriam-Webster be closest to the mark by calling avast an imperative verb. For 'tis true that most o' the time we say avast when givin' orders. But avast also can be used as a non-finite verb: For example, Melville uses it as an infinitive in chapter 40 of Moby Dick:

DUTCH SAILOR. Grand snoozing to-night, maty; fat night for that. I mark this in our old Mogul's wine; it's quite as deadening to some as filliping to others. We sing; they sleep--aye, lie down there, like ground-tier butts. At 'em again! There, take this copper-pump, and hail 'em through it. Tell 'em to avast dreaming of their lasses. Tell 'em it's the resurrection; they must kiss their last, and come to judgment.

And it can be the complement of a modal verb, like must or will: From The Cape diary and letters of William Mann, astronomer and mountaineer (1839), p. 21:

"Well, I must avast here & tell you that I went on board & adorned myself, ashore again & called at Mr Thompsons house to go along with him to the Admirals"

From "The Mids of Other Days", by Jonathan Oldjunk, in The United Service Journal and Naval and Military Magazine (1841), p. 64:

"By the powers! but I've no commands for that same," returned the master-at-arms, with whom Stewart was no especial favourite; "my only orders were to report to the docther; but if Musther Clarke will avast sheeting home till I come back, may be it's orders I shall get;" and he turned from the berth.

So avast is not just an imperative verb, but 'tis not used (except by landlubbers that have had a wee too much rum) in inflected forms (*he avasts, *they are avasting, etc.). 'Tis a defective verb like beware, which also can be used as an imperative ("Beware the jaberwock, my son") or as a non-finite form ("Ye must beware o' the black spot"). Arrh!