“We’ve just had our second grandchild.” You can respond to this good news with “Mazeltov, ken yirbu!” Ken yirbu means, “Thus should they multiply”. It is a blessing for more similar glad tidings. The expression appears in Rashi on the verses (Deuteronomy 11:18-21) “Impress these words on your very heart… and inscribe them on the doorposts of your house… so that you and your children’s days may multiply (yirbu).” Rashi explains, “If you do this, so they will multiply (ken yirbu).”
In addition to meaning “thus”, ken also means valid. Standing before Joseph in Egypt, the brothers declare their honest intentions to buy food and not spy out the land with the words, (Genesis 42: 11) “kenim anachnu” — “We are honest men!”
God validates the daughters of Zelophehad‘s request for a plot of land in Israel with the words (Deuteronomy 27:7), “Ken speak the daughters of Zelophehad.” Their claim is just. It was only in the Middle Ages that “ken” came to mean “yes”. People use “ken yirbu” as a blessing for more of any of good thing. But it is most commonly used regarding children.