Whereas a modern web software application has separate layers for presentation (user interface), business logic, and storage etc. (because modular construction is easier to build and debug) it is usually called an "n-tier architecture", where n represents the number of modules or layers. It is much more secure and robust than the "old way" (1-tier), where one machine was the web server, file server, database, and firewall. A program which has illogical or insufficient rules (i.e. absence of business logic) can be termed "(n-1) tier", as a crucial part (usually the part that makes the software smart or helpful) has obviously been omitted by scatterbrained developers, detached managers, clueless requirements analysts, dumb pilot members, etc.
Employee A: Did you submit your travel costs yet?
Employee B: No, our stupid online expense system kept giving me a cryptic error.
Employee A: Yup, that EOM app is an (n-1) tier system...
Boss: I need you to fix your time charges for last week. You entered 45 hours instead of 4.5 hours on Wednesday.
Subject: Must've been a fat-finger. Too bad our accounting system can't catch that obvious error. It's just another (n-1) tier waste of code.