Plainly is the same adverb used in 7.4. Though in other passages it may have the meaning of “in public” or “openly,” it obviously has the meaning of plainly in the present verse (Elsewhere in John’s Gospel this adverb is used in 7.4,13,26; 10.24; 11.14,54; 16.25,29; 18.20.) In some languages “openly” may be best translated “without hiding anything” or “without covering up the words,” while in others it is best to say “Jesus told them straight” or “Jesus told them so that they could understand well.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .