To have enjoyable sex(more enjoyable than normal that is)
Someone is sextiggerous if you can have enjoyable sex with them
If someone looks sextiggerous they appear like you could have enjoyable sex with them
<after a great session> wow that was sextiggerous!
or <when you see a previous conquest>Yeah he/she is sextiggerous
or <you see a hot boy/girl> He/She's hot, looks sextiggerous