1 made so that water cannot get in or out 不渗水的；不透水的: a watertight container 不透水的容器

2 (used about an excuse, opinion, etc) impossible to show to be wrong; without any faults （指借口、看法等）没有漏洞的，毫无破绽的: His alibi was absolutely watertight. 案发时他不在现场的证据毫无破绽。