MacBook Pro will not delete ALL its history
I'm doing some online Christmas shopping and don't want anyone in the family who shares this computer to see my online searches for their gifts. But when I went to delete my Safari history today, I noticed that it would not delete ALL the history despite my selecting the option to "clear all history". I also cannot delete the individual websites manually. What's even stranger is that there are several folders in History (8 different dates from the past month) that also will not delete. Some of the websites in those folders that will not delete are Yahoo mail, Facebook, Bing, Etsy, etc. Oddly enough, one of the dated folders has a date from Dec. 21, 2000 and I did not even own this MacBook until 2014 or so. Any advice on how I can get my MacBook to completely delete its history would be much appreciated. Thanks.
MacBook Pro Retina running MacOS Sierra Version 10.12.6/Safari Version 11.0)