"Fever 103°" begins with two linked rhetorical questions, in which the speaker questions the nature of purity. She suffers from a high fever throughout the poem, so one sense of the word "pure" here is "free of any contamination"—in other words, free of sickness. Taking "pure" in this sense, it is as if the speaker is asking whether or not she will ever be cured. However, the reference to "tongues of hell" in line 2 brings up a second meaning of "pure": "wholesome and untainted by immorality, especially of a sexual nature." The speaker, it seems, is overwhelmed with guilt at her own sins, fearing the punishment of hell, to the extent that she questions whether she even understands the word "pure" anymore.
Lines 2-5 contain an extended metaphor comparing fever's waves of heat with the "tongues of hell." "Tongues," in this sense, means flames. But as with line 1, there is a moral meaning to these words in addition to their literal meaning: hell delivers punishment to those who commit moral sins, and "tongues" can mean whips. The speaker's highly self-critical guilt, which she uses her own literal "tongue" to voice, is so wounding as to be comparable to being beaten with flaming whips in hell. But line 3 offers a further twist: rather than being razor sharp, as one would expect, these tongues are as "dull" (meaning blunt) as the tongues of the three-headed dog Cerberus, who guarded the gates to the underworld in Ancient Greek myth. This description implies that the speaker's guilt actually has little effect in purifying her of her sins. Epizeuxis in this section ("dull, dull"; "the sin, the sin") emphasizes the repetitive yet futile nature of the speaker's self-criticism and obsession with sin. The allusion to Cerberus, and the surprising description of a terrifying beast as "dull," "wheez[ing]", and "fat," serve to further heighten the sense of futility. It is as if the speaker is only just inside the gates of hell, exhausted but still not suffering enough—she needs to go further and to suffer more, if she is to achieve purification.
Lines 5-7 summarize these ideas of futility: "tongues" are "incapable" (meaning unable) of "licking clean" the speaker's "aguey tendon" (feverish muscles) or her "sin." Again thinking of "tongues" as referring to the speaker's own voice, these lines suggest that the speaker may be worried that even writing this poem won't do anything to cleanse her sin. Moreover, the fact that both "aguey tendon" and "the sin, the sin" are objects of the same verb phrase, "licking clean," further emphasizes the closeness between the physical sensations of sickness and the mental torment of guilt.
The most common metrical foot in these lines is the iamb (da-dum)—for instance, "The tongues of hell." But two exceptions, where two stressed syllables follow one another, break up this pattern. In both cases, the two stressed words fall on either side of a caesura: "dull, dull" and "dull, fat." Three of these four words are the same, so the shift in meter highlights the repetitive dullness of fever and of guilt, while "fat" likewise has connotations of slowness and ineffectiveness.