The Messenger of Allah (ﷺ) gave away a sheep. Aaishah (RA) asked the servant what people said. He said they made this dua:
بَارَكَ اللَّهُ فِيكُمْ
Transliteration
BaarakAllahu feekum
Translation
May Allah bless you all.
Aaishah (RA) then said:
وَفِيهِمْ بَارَكَ اللَّهُ
Transliteration
Wa feehim baarakAllah
Translation
And may Allah bless them.
So we return their dua in the same way, and our reward stays with us:
وَفِيكَ بَارَكَ اللَّهُ
Transliteration
Wa feeka baarakAllah
Translation
And May Allah bless you.
Say wa feek barak Allah - it means may Allah bless you too. Wa iyyak or ameen wa feek also work. All these replies return the blessing nicely.
May Allah bless her - that's what barakallahu laha means. For boys, use Allahumma Barik Lahu. For things, say barakallahu laka fiha. Each asks Allah for blessings.
Men hear baarakallahu feeka. Women hear baarakallahu feeki. Baarakallahu feek works for one person. People use it every day.
Reply with wa iyyak or barakallahu feek. Want to thank them? Say jazākAllahu khayran instead.
Say it to thank and bless someone. It's a good reply to jazakallah or jazakallah khayran. It thanks and asks Allah for blessings at the same time.