My friend Mark Ward often points out “false friends” in the KJV—words and phrases in the KJV that contemporary readers are likely to think they understand but in fact misunderstand due to changes in language.
The KJV of Genesis 1:28 begins, “Be fruitful, and multiply, and replenish the earth.” In contemporary English, “replenish” means to fill again (and, according to the OED, it had this sense in the 1600s as well). But the OED also indicates that at the time the KJV was translated, “replenish” could bear a different meaning: “fill abundantly” (again, see the OED). This is certainly the meaning intended in this context.
Interestingly, the Wycliffe, Coverdale, and Geneva Bibles all use the word “fill” here.