Definition of botom pair
The botom pair is the smallest of the pairs. The expression is used in relation to a flop and not in relation to a pair that one would receive in hand before the flop. Let's imagine a flop A-5-9. If you have AK in your hand, you have "top pair", the best pair. If you have K9 in your hand, you have "middle pair", the middle pair, and if you have K5 (or any hand with a 5), you have "botom pair". If the turn brings a 2 or a card smaller than a 5, you will now have "middle pair" or a middle pair.
botom pair, botom set, botom full house, etc.
The word "botom" also applies to other types of hands. To use the same flop as above, if you have AA, you will have the top three of king or "top set" in English. If you have 99, you will have "middle three of king" or "middle set" in English. And finally, if you have 55, you will have botom sets. We also distinguish the different types of full hand. For example, on a board A-A-K-5-2, a player with 22 in hand would have "botom full house", which is the smallest possible full hand on that board.