There is no direct English translation for this Korean word.
saekki literally means baby, and you can hear it affectionally spoken by a grandmother.
But it’s also used as an insult, especially with the word gae (=dog) before it, which makes it into “son of a dog”. Koreans had to invent a different word for when they actually mean “puppy”, baby dog.
Most of the time it is translated as “jerk” or “■■■■■■■”, depending on the seriousness of the situation. But it is an insult, make no mistake about it!
You may want to read a more in-depth analysis about how Viki translating teams view insults and swearwords in the following discussions, where many respected Viki veterans took part and some of them being Korean gave us a more rounded view of things.