Alexander Dyce, A General Glossary to Shakespeare's Works

fig “of Spain!—The,” HENRY V., iii. 6. 58. Here “Pistol, after spurting out his ‘figo [fico] for thy friendship’ [see fico, etc.]; as if he were not satisfied with the measure of the contempt expressed, more emphatically adds, ‘the fig of Spain.’ This undoubtedly alludes to the poisoned figs mentioned in Mr. Steevens's note, because [as Steevens observes] the quartos read ‘the fig of Spain within thy jaw,’ and ‘the fig within thy bowels and thy dirty maw.’ Or, as in many other instances, the allusion may be twofold; for the Spanish fig, as a term of contempt only [see the preceding article], must have been very familiar in England in Shakspeare's time” (DOUCE) . In the note to which Douce refers above, Steevens, to illustrate “the custom of giving poisoned figs to those who were the objects either of Spanish or Italian revenge,” cites, among other passages:
“I do look now for a Spanish fig, or an Italian salad, daily.”
Webster's White Devil,— Works, p. 30,
ed. Dyce, 1857;
“I must poison him;
One fig sends him to Erebus.”
Shirley's Brothers— Works, vol. i. p. 231,
ed. Gifford and Dyce.