The first two lines of "Cut" present what seems, at first, like a pretty straightforward situation: the speaker, attempting to chop "an onion," has accidentally sliced into her "thumb instead." While the poem will go on to describe the speaker's emotional reaction to this cut in vivid detail, the incident itself is really only gestured to in the title and these opening lines.
The speaker's tone at the poem's start seems ironic. Though she calls cutting her thumb a "thrill" she probably isn't really deriving any pleasure from this accidental maiming. Her wry humor perhaps suggests that she's attempting to distance herself from the very real pain that she's in. The word "thrill" also points to the surreal mixture of surprise, confusion, and maybe even excitement that comes with suddenly, accidentally injuring oneself. This reminder of her fragile, mortal body is perhaps thrilling amidst the humdrum of everyday life.
Then again, the poem never actually says that this injury was an accident! The speaker may very well have intentionally cut herself, either consciously or unconsciously trying to resolve some inner conflict.
In any case, the speaker appears to be in shock. The /uh/ assonance and muffled /m/ and /n/ sounds in "thumb" and "onion" evoke a stunned numbness, as if the speaker hasn't fully processed what has happened to her. The soft /th/ alliteration in "thrill" and "thumb" also adds immediate intensity to the poem's opening.
Note, too, how the dash at the end of line 1 interrupts the speaker's thought, giving it a truncated feel that subtly mimics the way the speaker's thumb itself has been cut short:
What a thrill—
Indeed, the poem consists mostly of short lines arranged in neat quatrains (four-line stanzas). These short lines and stanzas grant the poem a choppy (pun intended!) feel that evokes not only the speaker's mangled thumb, but also, perhaps, the sharp abbreviation of her thoughts and feelings.