In Act 3, Scene 2 of As You Like It, Celia tells Rosalind that Orlando is in the Forest of Arden and is posting love poems about her on all the trees. The news makes Rosalind overwhelmed with emotions that betray how much she loves Orlando. She wants Celia to tell her everything at once.

Alas the day! what shall I do with my doublet and hose?--
What did he when thou saw'st him? What said he? How look'd he?
Wherein went he? What makes he here? Did he ask for me? Where
remains he? How parted he with thee? and when shalt thou see
him again? Answer me in one word.

Rosalind does not mean "Answer me in one word" literally. She means that she wants immediate answers to all those questions. But Celia pretends to take her literally and says:

You must borrow me Gargantua's mouth first: 'tis a word too
great for any mouth of this age's size.

Gargantua is a giant who is featured in a series of French novels titled La vie de Gargantua et de Pantagruel (The Life of Gargantua and Pantagruel) by the French writer Francois Rabelais. Pantagruel is Gargantua's son. The five books were published in the sixteenth century between 1532 and 1564. They were popular largely because of their vulgarity, obscenity, and impiety. (See e-notes reference links below.) The year 1564 happens to be the year William Shakespeare was born. 

Celia, of course, is saying, that she would need the mouth of a giant in order to be able to utter a single word that would answer all of Rosalind's questions about Orlando at once.